OBJECTIVES 1.Describe transaminase, and glutamate dehydrogenase reactions; discuss their roles in the removal of nitrogen waste in the body. 2.In relation to the urea cycle: a)describe the carbamoyl phosphate synthetase I and ornithine transcarbamoylase reactions b)identify direct sources of nitrogen for the urea cycle c) describe roles of acetylglutamate and arginine in regulation of the cycle 3.In relation to hyperammonemia a)define the term b)discuss why a defect in either carbamoyl phosphate synthetase I or ornithine transcarbamoylase causes this problem c)explain how excess ammonia might cause an energy deficit in brain cells.
OBJECTIVES (cont.) 4.List the key endproduct(s) for catabolism of each of the following amino acids: alanine; aspartate; glutamate; glutamine; isoleucine; leucine; phenylalanine; tyrosine; valine 5.For phenylalanine hydroxylase: a)describe the reaction b)explain its relationship to phenylketonuria; c)describe unusual side reactions of excess phenylalanine d)discuss why an individual may be mentally retarded if the enzyme is lacking
-Amino acid -Keto acid NH 2 HOOC-CH-CH 2 CH 2 COOH O HOOC-C-R NH 2 HOOC-CH-R O HOOC-C-CH 2 CH 2 COOH -Ketoglutarate Glutamate Cofactor = pyridoxal phosphate Figure 1. Depiction of a general transamination (aminotransferase) reaction. The -amino acid other than glutamate can be a wide variety
+ -ketoglutarate + glutamate Aspartate aminotransferase (glutamate-oxaloacetate transaminase) NH 2 Aspartate HOOC-CH-CH 2 COOH O Oxaloacetate HOOC-C-CH 2 COOH Alanine aminotransferase (glutamate-pyruvate transaminase) + -ketoglutarate + glutamate NH 2 Alanine HOOC-CH-CH 3 O Pyruvate HOOC-C-CH 3 Figure 2. The reactions catalyzed by aspartate aminotransferase and alanine aminotransferase.
NADH NAD + -Ketoglutarate + NH 4 + Glutamate Glutamate dehydrogenase Glutamine Glutamine synthetase NH 3 + ATP ADP + P i Figure 3. In non-hepatic tissues the linked reactions of glutamate dehydrogenase and glutamine synthetase remove two ammonia molecules from the tissues as a way of ridding the tissues of nitrogen waste. The glutamine deposits the ammonia in the kidney for excretion.
Figure 4. In liver, nitrogen waste from amino acids ends up in urea. Amino acids are derived either from the breakdown of protein in various tissues or from what is synthesized in those tissues -Amino acid -Keto acid -Ketoglutarate Glutamate Aminotransferase NAD + + H 2 O Glu dehydrogenase -Ketoglutarate Glutamate NADH + NH 4 + NH 4 + UREA Urea cycle
CYTOPLASM MITOCHONDRIA Figure 5. Carbamoyl phosphate synthetase reaction and the urea cycle. Overall: 3ATP+HCO 3 - +NH 4 + +asp 2ADP+AMP+2P i +PP i +fumarate+urea Ornithine Citrulline argininosuccinate synthetase argininosuccinase arginase AMP+PP i - Aspartate Argininosuccinate ATP Arginine Fumarate (returns to TCA cycle) PiPi Ornithine Citrulline Ornithine transcarbamoylase Carbamoyl phosphate 2ATP + HCO 3 - + NH 4 + 2ADP + P i Carbamoyl phosphate synthetase UREA O H 2 N-C- NH 2 Ornithine - OOC-CH- NH 3 + CH 2 COO -
UREA CYCLE FACTS Found primarily in liver and lesser extent in kidney Nitrogen added to the urea cycle via carbamoyl phosphate and aspartate Carbamoyl phosphate synthetase is allosterically activated by N-acetylglutamate (acetyl CoA + glutamate N-acetylglutamate) Arginine stimulates the formation of N-acetylglutamate
Fatty liver that can lead to cirrhosis HYPERAMMONEMIAS Acquired = Liver disease leads to portal-systemic shunting Inherited = Urea cycle enzyme defects of CPS I or ornithine transcarbamoylase lead to severe hyperammonemia
GLUCOSE Pyruvate Oxaloacetate Citrate Fumarate Acetyl CoA Malate -Ketoglutarate Succinyl CoA Succinate Isocitrate Alanine, Serine, Glycine, Threonine Glutamate Glutamine Proline Figure 6. Fates of carbons from degradation of amino acids Aspartate Asparagine Leucine, Isoleucine, Phenylalanine, Tyrosine Phenylalanine Tyrosine Methionine Isoleucine Valine
O2O2 Tyrosine H2OH2O Dihydrobiopterin Phenylalanine hydroxylase Phenylalanine NADP + NADPH Tetrahydrobiopterin Figure 7. The phenylalanine hydroxylase reaction and its cofactor